Method and system for distributing commands to targets

ABSTRACT

Embodiments of systems and methods for routing commands to a distributed RAID system are disclosed. Specifically, embodiments may route a command to an appropriate data bank in a distributed RAID system, where each data bank has a set of associated storage media and executes a similar distributed RAID application. The distributed RAID applications on each of the data banks coordinate among themselves to distribute and control data flow associated with implementing a level of RAID in conjunction with data stored on the associated storage media of the data banks.

RELATED APPLICATIONS

This application is a continuation of and claims a benefit of priorityunder 35 U.S.C. 120 of the filing date of U.S. patent application Ser.No. 12/479,403, by inventors Galloway et al., entitled “Method andSystem for Distributing Commands to Targets” filed on Jun. 5, 2009,which in turn claims a benefit of priority under 35 U.S.C. §119 toprovisional patent application Nos. 61/131,270 by inventors Galloway etal., entitled “Method and System for Distributed RAID Implementation”filed on Jun. 6, 2008; and 61/131,314 by inventors Galloway et al.,entitled “Method and System for Data Migration in a DistributedMulti-Processor RAID Subsystem” filed Jun. 6, 2008; and 61/131,291 byinventors Galloway et al., entitled “System and Method for DistributingRead/Write Requests to Optimal SCSI Targets” filed Jun. 6, 2008; and61/131,290 by inventors Galloway et al., entitled “Method and System forUtilizing Storage in a Storage System” filed Jun. 6, 2008; and61/131,379 by inventors Galloway et al., entitled “Method and System forRebuilding Data” filed Jun. 6, 2008; and 61/131,312 by inventorsGalloway et al., entitled “Method and System for Placement of Data onStorage” filed Jun. 6, 2008; the entire contents of each are herebyfully incorporated by reference herein for all purposes.

TECHNICAL FIELD

This invention relates generally to the use of storage devices. Moreparticularly, embodiments of this invention relate to implementing RAIDon storage devices. Even more specifically, certain embodiments of thisinvention relate to a distributed implementation of RAID.

BACKGROUND

Data represents a significant asset for many entities. Consequently,data loss, whether accidental or caused by malicious activity, can becostly in terms of wasted manpower, loss of goodwill from customers,loss of time and potential legal liability. To ensure proper protectionof data for business, legal or other purposes, many entities may desireto protect their data using a variety of techniques, including datastorage, redundancy, security, etc. These techniques may, however,conflict with other competing constraints or demands imposed by thestate or configuration of computing devices used to process or storethis data.

One method for dealing with these tensions is to implement a RedundantArray of Independent Disks (RAID). Generally, RAID systems divide andreplicate data across multiple hard disk drives (or other types ofstorage media), collectively referred to as an array, to increasereliability and in some cases improve throughput of computing devices(known as a host) using these RAID systems for storage. To a host then,a RAID array may appear as one or more monolithic storage areas. When ahost desires to communicate (read, write, etc.) with the RAID system thehost communicates as if the RAID array were a single disk. The RAIDsystem, in turn, processes these communications to implement a certainRAID level in conjunction with such communications. These RAID levelsmay be designed to achieve some desired balance between a variety oftradeoffs such as reliability, capacity, speed, etc. For example, RAID(level) 0 distributes data across several disks in a way which givesimproved speed and utilizes substantially the full capacity of thedisks, but all data on a disk will be lost if the disk fails; RAID(level) 1 uses two (or more) disks which each store the same data, sothat data is not lost so long as one disk survives. Total capacity ofthe array is substantially the capacity of a single disk and RAID(level) 5 combines three or more disks in a way that protects dataagainst loss of any one disk; the storage capacity of the array isreduced by one disk.

Current implementations of RAID may have a variety of problems. Theseproblems may stem from limitations imposed by the architecture of theseRAID systems, such as the fact that in many instances all communicationswith a RAID system must be addressed to a single server which controlsand manages the RAID system. Other problems may arise from theconfiguration or layout of the data on the disks comprising a RAIDsystem. For example, in certain cases a RAID level must be chosen andstorage allocated within the RAID system before the RAID system can beutilized. Thus, the initially chosen RAID level must be implemented inconjunction with the data stored on the RAID system, irrespective ofwhether that level of RAID is desired or needed. In many cases theseexisting problems may be exacerbated by the need to use custom hardwareor software to implement these solutions, raising the costs associatedwith implementing such a solution.

RAID controllers can direct host generated read and write requests whichare addressed to logical units to RAID data banks underlying the logicalunits. RAID controllers also manage failure of RAID data banks and RAIDnetworks for hosts. Hosts, therefore, remain unburdened by aspects ofRAID architecture and operation. RAID controllers, however, can act aspotential single point failures and can act as bottle-necks, therebyintroducing undesirable latency into read and write request handling.

Consequently, it is desired to substantially ameliorate these problems.

SUMMARY

Embodiments of systems and methods for routing commands to a distributedRAID system are disclosed. Specifically, embodiments may route a commandto an appropriate data bank in a distributed RAID system, where eachdata bank has a set of associated storage media and executes a similardistributed RAID application. The distributed RAID applications on eachof the data banks coordinate among themselves to distribute and controldata flow associated with implementing a level of RAID in conjunctionwith data stored on the associated storage media of the data banks.

One embodiment provides hosts with device specific modules (DSM)configured to direct READ and WRITE commands from the hosts to RAID databanks. Various hosts and data banks can communicate over SCSI or iSCSImulti-pathing networks. Hosts can include multi-pathing input/output(MPIO) device drivers with which DSMs can cooperate to route READ andWRITE commands to one or more data banks. Hosts can use logical blockaddresses (logical addresses) to designate target addresses for variousREAD and WRITE commands. DSMs can examine various READ and WRITEcommands, parse target logical addresses from the commands, and maplogical addresses to data banks. DSMs can route READ and WRITE commandsto RAID data banks associated with logical addresses.

Aspects and embodiments of the invention may allow data banks to manageREAD and WRITE commands without intervening RAID controllers andassociated latency penalties. Embodiments may thus reduce RAID subsystemcost and complexity by allowing hosts to communicate with RAIDsubsystems without utilizing any intervening RAID controllers thusreducing latency by allowing hosts to read to and write from data banksdirectly.

These, and other, aspects of the invention will be better appreciatedand understood when considered in conjunction with the followingdescription and the accompanying drawings. The following description,while indicating various embodiments of the invention and numerousspecific details thereof, is given by way of illustration and not oflimitation. Many substitutions, modifications, additions orrearrangements may be made within the scope of the invention, and theinvention includes all such substitutions, modifications, additions orrearrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerimpression of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore nonlimiting, embodimentsillustrated in the drawings, wherein identical reference numeralsdesignate the same components. Note that the features illustrated in thedrawings are not necessarily drawn to scale.

FIG. 1 is a block diagram of one embodiment of an architecture employinga distributed RAID system.

FIG. 2A is a block diagram of one embodiment of a data bank.

FIG. 2B is a block diagram of one embodiment of an architecture for adata bank.

FIG. 3 is a flow diagram of one embodiment of a method implemented by adistributed RAID system.

FIG. 4 is block diagram of an example of one embodiment of anarchitecture employing a distributed RAID system.

FIG. 5 is a block diagram of one embodiment of a table.

FIG. 6 is a block diagram of one embodiment of a table.

FIG. 7 is a block diagram of one embodiment of a table.

FIG. 8 is a flow diagram of one embodiment of a method implemented by anapplication on a host.

FIG. 9 is a block diagram of one embodiment of a table.

DETAILED DESCRIPTION

The invention and the various features and advantageous details thereofare explained more fully with reference to the nonlimiting embodimentsthat are illustrated in the accompanying drawings and detailed in thefollowing description. Descriptions of well known starting materials,processing techniques, components and equipment are omitted so as not tounnecessarily obscure the invention in detail. It should be understood,however, that the detailed description and the specific examples, whileindicating preferred embodiments of the invention, are given by way ofillustration only and not by way of limitation. Various substitutions,modifications, additions and/or rearrangements within the spirit and/orscope of the underlying inventive concept will become apparent to thoseskilled in the art from this disclosure. Embodiments discussed hereincan be implemented in suitable computer-executable instructions that mayreside on a computer readable medium (e.g., a HD), hardware circuitry orthe like, or any combination.

Before discussing specific embodiments, embodiments of a hardwarearchitecture for implementing certain embodiments is described herein.One embodiment can include one or more computers communicatively coupledto a network. As is known to those skilled in the art, the computer caninclude a central processing unit (“CPU”), at least one read-only memory(“ROM”), at least one random access memory (“RAM”), at least one harddrive (“HD”), and one or more input/output (“I/O”) device(s). The I/Odevices can include a keyboard, monitor, printer, electronic pointingdevice (such as a mouse, trackball, stylist, etc.), or the like. Invarious embodiments, the computer has access to at least one databaseover the network.

ROM, RAM, and HD are computer memories for storing computer-executableinstructions executable by the CPU. Within this disclosure, the term“computer-readable medium” is not limited to ROM, RAM, and HD and caninclude any type of data storage medium that can be read by a processor.In some embodiments, a computer-readable medium may refer to a datacartridge, a data backup magnetic tape, a floppy diskette, a flashmemory drive, an optical data storage drive, a CD-ROM, ROM, RAM, HD, orthe like.

At least portions of the functionalities or processes described hereincan be implemented in suitable computer-executable instructions. Thecomputer-executable instructions may be stored as software codecomponents or modules on one or more computer readable media (such asnon-volatile memories, volatile memories, DASD arrays, magnetic tapes,floppy diskettes, hard drives, optical storage devices, etc. or anyother appropriate computer-readable readable medium or storage device).In one embodiment, the computer-executable instructions may includelines of complied C++, Java, HTML, or any other programming or scriptingcode.

Additionally, the functions of the disclosed embodiments may beimplemented on one computer or shared/distributed among two or morecomputers in or across a network. Communications between computersimplementing embodiments can be accomplished using any electronic,optical, radio frequency signals, or other suitable methods and tools ofcommunication in compliance with known network protocols.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,process, article, or apparatus that comprises a list of elements is notnecessarily limited only those elements but may include other elementsnot expressly listed or inherent to such process, process, article, orapparatus. Further, unless expressly stated to the contrary, “or” refersto an inclusive or and not to an exclusive or. For example, a conditionA or B is satisfied by any one of the following: A is true (or present)and B is false (or not present), A is false (or not present) and B istrue (or present), and both A and B are true (or present).

Additionally, any examples or illustrations given herein are not to beregarded in any way as restrictions on, limits to, or expressdefinitions of, any term or terms with which they are utilized. Instead,these examples or illustrations are to be regarded as being describedwith respect to one particular embodiment and as illustrative only.Those of ordinary skill in the art will appreciate that any term orterms with which these examples or illustrations are utilized willencompass other embodiments which may or may not be given therewith orelsewhere in the specification and all such embodiments are intended tobe included within the scope of that term or terms. Language designatingsuch nonlimiting examples and illustrations includes, but is not limitedto: “for example”,“for instance”, “e.g.”, “in one embodiment”.

This application is related to U.S. patent application Ser. No.12/479,319 entitled “Method and System for Distributed RAIDImplementation” by Galloway et al., filed on Jun. 5, 2009; Ser. No.12/479,360, entitled “Method and System for Data Migration in aDistributed RAID Implementation” by Galloway et al., filed on Jun. 5,2009; Ser. No. 12/479,377, entitled “Method and System for InitializingStorage in a Storage System” by Galloway et al., filed Jun. 5, 2009;Ser. No. 12/479,434, entitled “Method and System for Rebuilding Data ina Distributed RAID System” by Galloway et al., filed Jun. 5, 2009; andSer. No. 12/479,394, entitled “Method and System for Placement of Dataon a Storage Device” by Galloway et al., filed Jun. 5, 2009 all of whichare incorporated fully herein by reference.

A brief discussion of context particularly with respect to data storagemay now be helpful. As discussed above, RAID systems divide andreplicate data across multiple hard disk drives (or other types ofstorage media), collectively referred to as an array, to increasereliability and in some cases improve throughput of computing devices(known as a host) using these RAID systems for storage. However, currentimplementations of RAID may have a variety of problems. These problemsmay stem from limitations imposed by the architecture of these RAIDsystems, from the configuration or layout of the data on the diskscomprising a RAID system or from the need to use custom hardware orsoftware to implement these solutions, raising the costs associated withimplementing such a solution. It is desired to substantially amelioratethese problems, among others.

To that end, attention is now directed to the systems and methods of thepresent invention. Embodiments of these systems and methods provide adistributed RAID system comprising a set of data banks. Moreparticularly, in certain embodiments of a distributed RAID system eachdata bank has a set of associated storage media and executes a similardistributed RAID application. The distributed RAID applications on eachof the data banks coordinate among themselves to distribute and controldata flow associated with implementing a level of RAID in conjunctionwith data stored on the associated storage media of the data banks.

Specifically, in certain embodiments, a volume with an associated RAIDlevel may be created using the distributed RAID system. Each of thedistributed RAID applications can then coordinate operations associatedwith data of that volume such that data associated with that volume orthe implementation of the desired RAID level in conjunction with thatvolume may be stored on the multiple data banks of the distributed RAIDsystem.

By coordinating the implementation of a level of RAID in conjunctionwith a volume by storing both data of the volume and data associatedwith the implementation of RAID on multiple data banks using similardistributed RAID applications executing on each of those data banks anumber of advantages may be achieved. Namely, different storage volumesmay be allotted, with one or more of the volumes implemented inconjunction with different RAID levels. Moreover, as the coordination ofstorage and the implementation of RAID across the data banks isaccomplished using substantially identical distributed RAIDapplications, in many cases standard or off-the-shelf hardware, such asstandard x86 based servers and storage media may be utilized. Many otheradvantages may also be realized utilizing embodiments presented hereinor other embodiments, and such advantages, which may or may not bepointed out in particular detail, will be realized after reading thisdisclosure.

Turning now to FIG. 1, a block diagram of an architecture for a systemwhich utilizes one embodiment of a distributed RAID system is depicted.Distributed RAID system 100 includes a set of data banks 110, each databank 110 communicatively coupled to both of switches 120. Each ofswitches 120 is also communicatively coupled to each host 102, such thata host 102 may communicate with each data bank 110 through a set ofpaths corresponding to a particular data bank 110, each path comprisingone of the switches 120.

The communicative coupling between data banks 110, switches 120 andhosts 102 may be accomplished using almost any transport medium (eitherwired or wireless) desired, including Ethernet, SCSI, iSCSI, FibreChannel, serial attached SCSI (“SAS”), advanced technology attachment(“ATA”), serial ATA (“SATA”) or other protocols known in the art.Furthermore, the communicative coupling may be implemented inconjunction with a communications network such as the Internet, a LAN, aWAN, a wireless network or any other communications network known in theart.

In one embodiment, then, using a commands protocol, such as iSCSI, SCSI,etc., hosts 102 may communicate with data banks 110 to manipulate data.More particularly, each of data banks 110 comprises storage media (aswill be explained in more detail later on herein). Collectively, thestorage media in data banks 110 may be virtualized and presented tohosts 102 as one or more contiguous blocks of storage, storage devices,etc. For example, when the iSCSI protocol is utilized the storage mediain data banks 110 may be presented to hosts 102 as a SCSI target with,in one embodiment, multiple ports.

Thus, during operation, in one embodiment a host 102 (or a user at ahost 102 or interfacing with data bank 110) may request the creation ofa volume and specify a level of RAID to be implemented in conjunctionwith that volume. Data associated with that volume and theimplementation of the desired level RAID in association with that volumeis stored across data banks 110. The hosts 102 may then access thisvolume using logical address corresponding to the volume or a portionthereof. In this manner, hosts 102 can utilize created volumes ofstorage and fault tolerance can be achieved in conjunction with thesevolumes substantially invisibly to hosts 102.

The virtualization of storage and the implementation of RAID utilizingdata banks 110 may be better understood with reference to FIG. 2A whichdepicts a block diagram of one embodiment of a data bank 110 computeroperable to implement distributed RAID. Here, data bank 110 comprises adata store 250, and a processor 202 operable to execute instructionsstored on a computer readable medium, where the instructions areoperable to implement distributed RAID application 210. Distributed RAIDapplication 210 may periodically issue heartbeat communications todistributed RAID applications 210 on other data banks 110 to determineif there has been a fault with respect to that data bank 110. If thedistributed RAID application 210 determines that another data bank 110is experiencing a fault it may set one or more fault flags correspondingto that data bank 110. Using these fault flags for each distributed RAIDapplication 210 on each data bank 110 a particular distributed RAIDapplication 210 may determine if a certain data bank 110 is faulty.

Distributed RAID application 210 may also have access (for example, toread, write, issue commands, etc.) to data store 250 comprising one ormore storage media, which may for example be disks 252 operatingaccording to almost any protocol known, such as SATA, PATA, FC, etc.where each of the disks 252 may, or may not, be of equal size.Distributed RAID application 210, executing on each of data banks 110can allow the allocation of and use of volumes using data stores 250across data banks 110 and the implementation of RAID in conjunction withthese volumes utilizing a set of global tables 240 shared between databanks 110, a set of local tables 245 and write cache 260, all of whichmay be stored in a memory 230 (which may be data store 250 or anothermemory altogether).

FIG. 2B depicts a block diagram of one embodiment of a hardwarearchitecture which may be used to implement data bank 110 computeroperable to implement distributed RAID. In this architectural example,data bank 110 comprises one or more processors 202 which may adhere tothe Intel x86 architecture or some other architecture altogether and amemory 230 coupled through a bus to I/O controller hub 212, which in oneembodiment may be a southbridge chip or the like. The I/O controller hub212 may, in turn, be coupled to and control a bus 272 such as a PCI-Xbus, PCI-express bus, etc. Coupled to this bus 272 are one or more diskcontrollers 262 such as, for example, an LSI 1068 SATA/SAS controller.Each of these disk controllers 262 is coupled to one or more disks 252,where collectively these disks 252 may comprise data store 250.Additionally, one or more network interfaces 282 may also be coupled tobus 272. These network interfaces 282 may be network interfaces (such asEthernet, etc.) which are included on motherboards, may comprise one ormore network interface cards configured to interface via one or moreprotocols such as Ethernet, fibre channel, etc. or may be some othertype of network interface such that data bank 110 may communicate withswitched 120 through these network interfaces 282.

Moving now to FIG. 3, one embodiment of a method for the allocation ofvolumes and the laying out of data associated with these volumes in datastores 250 across data banks 110 is illustrated. Initially, beforevolumes are to be allocated on a disk 252 of data store 250, the disk252 may be formatted at step 305. As discussed above, in order to havethe ability to easy and simply recover from any failures redundancy datamay need to be accurate relative to any corresponding stored data. Inmany cases, this may entail that disks 252 utilized to store a volume beformatted by calculating redundancy data from the current data in theareas on disk 252 where portions of a volume are to be stored, eventhough data stored at these areas may currently be garbage values. Thesecalculations may consume an undesirably large amount of time.

Furthermore, in a distributed RAID environment such as that detailedwith respect to FIG. 1, other problems may present themselves. Morespecifically, as different portions of a volume may be stored ondifferent data banks 110 and redundancy data corresponding to the volumemay also be stored on various data banks 110, accomplishing this type offormatting may additionally require a great deal of communicationbetween distributed RAID applications 210 on data banks 110, consumingprocessor cycles and communication bandwidth.

Thus, in one embodiment, to ensure that redundancy data corresponding toan area of a disk where data of a volume is to be stored is accuraterelative to the area of disk 252 where that data of the volume is to bestored, a zero value may be written to the areas on disks 252 where datacorresponding to the volume is to be stored and the areas on disk 252where redundancy data is to be stored. By zeroing out both the areas ofa disk 252 where data of a volume is to be stored and areas of disks 252where redundancy data is to be stored it can be guaranteed that any dataof the volume can be recreated from its corresponding redundancy data.

Zeroing disks 252 may have other advantages. Namely that no complexcalculations may need to be performed to determine redundancy data andno communications between distributed RAID applications 210 may be toachieve relative accuracy between areas where a volume is to be storedand redundancy data corresponding to those areas.

Importantly, by zeroing out areas of disks 252 for use with a volume andits corresponding redundancy data a significant delay in the usabilityof RAID system 100 may be avoided. These advantages may be attainedthrough the use of a process which substantially continuously duringoperation zeros out unallocated areas of disks 252 resulting, forexample, from the initial use of distributed RAID system 100, theinstallation of new disks 252, the deletion of a volume, etc. In theseinstances, currently unallocated (i.e. not currently allocated) areas ofdisks 252 on each of data banks 110 may have zeros written to them(referred to as “zeroing” the area).

The unallocated areas of disks 252 which have been zeroed may be trackedsuch that when a command corresponding to a portion of a volume orredundancy data associated with a portion of a volume is received at adata bank 110 to which that portion is assigned, distributed RAIDapplication 210 may check to determine if that portion has been assigneda corresponding area of disks 252 on data bank 110 where that portionhas been assigned. If no corresponding area of disks 252 on data bank110 has been assigned, distributed RAID application 210 may select anarea of disks 252 which has been zeroed and assign this area of disks252 to the portion of the volume or corresponding redundancy data.

By simultaneously zeroing out any unassigned areas which have notpreviously been zeroed and waiting until a command corresponding to aportion of a volume or redundancy data is received to assign a zeroedarea of disks 252 to that portion distributed RAID system 100 mayoperate substantially immediately without a long involved formattingprocess and new disks 252 may be added and volumes deleted or freedrelatively unobtrusively to the operation of distributed RAID system100.

It will be noted, therefore, after reading the above that step 305 inwhich the disks 252 are formatted may be accomplished before, during orafter the creation of a volume with respect to distributed RAID system100 and that the placement of step 305 (and all other steps in all theflow diagrams herein) implies no order to the steps. As will also benoted after a thorough review of the rest of the steps in FIG. 3 and theremainder of the disclosure, a volume may be created and portions of thevolume and redundancy data corresponding to the volume assigned to adata bank 110 before physical areas on disks 252 on these data banks 110have been assigned to store the portions of the volume or redundancydata and that, furthermore, the zeroing of the physical areas on disks252 used to store the data corresponding to such portions may occurbefore the creation of the volume or after the creation of the volumebut before these physical areas are assigned to corresponding portionsof the volume or redundancy data (as discussed in more detail later).

These locations may be better explained with reference to the concept ofa segment which may be utilized by embodiments of a distributed RAIDapplication 210, where a segment may be the size of 2048 logical blockaddresses (LEAs) (or some other size) and the size of the logical blockaddress corresponds to the sector size of a disk 252. Disks 252 in thedata store 250 on each of data banks 110 may therefore be separated intoequal size segments (for example, 1 MB) at step 310. These segments maycorrespond to one or more contiguous data blocks of a disk drive 252.Therefore, when a user or host 102 requests the creation of a volumefrom distributed RAID application 210 at step 320 and specifies a levelof RAID which will be used in conjunction with that volume at step 330,a number of these segments corresponding to the requested size of thevolume plus the number of segments desired to implement the desiredlevel of RAID in conjunction with the volume may be assigned to thevolume at step 340.

Thus, the volume comprises a number of segments (also referred to aslogical segments), where each of these segments may be associated with aparticular data bank 110 such that the data bank 110 may be assigned tomanage that segment of the volume. This segment may, for example, may bethe size of 2048 logical block addresses (LBAs), where the size of thelogical block address corresponds to the size sector size of a disk 252(other arrangements and sizes will also be possible). In most cases thephysical storage comprising that segment of the volume will be stored inthe data store 250 of the data bank 110 which manages that segment,however, in other cases the data corresponding to that segment may bestored in the data store 205 of a different data bank 110 (in otherwords, in certain cases the data bank 110 comprising the distributedRAID application which manages that segment may be distinct from thedata bank 110 comprising the data store 250 which stores the datacorresponding to that segment).

In one embodiment, the allocation of segments to data banks 110corresponding to a particular volume may be accomplished by determininga random permutation of the set of data banks 110 corresponding to thedistributed RAID system 100. Thus, if there are six data banks a randompermutation of size six, where the random permutation comprises each ofthe data banks may be determined such that the segments may assigned toeach of the data banks consecutively in the order of the randompermutation.

For example, suppose there are four data banks 110 in a distributed RAIDsystem (call them data bank1, data bank2, etc.). A random permutation ofdata bank2, data bank4, data bank1 and data bank3 may be determined. Inthis case, the first segment corresponding to a volume is on data bank2,the second segment may be on data bank4, the third on data bank1, thefourth on data bank 3 and the fifth back again on data bank 4. In thisway, the location of a particular segment corresponding with the volumemay be determined mathematically if the random permutation correspondingto the volume is known.

As mentioned the user may specify that a level of RAID is to beimplemented in conjunction with a volume at step 330. In this case,distributed RAID application 210 may ensure that any data correspondingto the implementation of RAID in conjunction with a volume is stored atan appropriate location at step 350 such that the RAID information isappropriately distributed across data banks 110 to ensure that thedesired level of RAID is achieved.

For example, if it is desired to implement RAID 5 in conjunction with avolume, distributed RAID application 210 may determine a desired RAIDparity group size (for example, based on a user configured RAID set orotherwise determined). This determination may be based on the number ofdata banks 110 in the distributed RAID system and may, in oneembodiment, be one less than the number of data banks 110 (plus anadditional one to account for the parity data).

To illustrate, if there were five data banks 110, for every foursegments which store data associated with the volume (referred to asdata segments), one segment would be dedicated to parity and the parityfor the four segments calculated and stored in this parity segment,where the parity segment would be dedicated in a data bank 110 whosedata store 250 does not comprise the data segments from which the paritydata of the parity segment was calculated.

At this point, each segment corresponding to a logical volume has beenassigned to a particular data bank 110 and any segments 100 to beutilized to store RAID data corresponding to the volume (referred toherein interchangeably as redundancy segments or parity segments,without loss of general applicability to the use of the segment to storeany type of redundancy data associated with the implementation of anylevel of RAID in conjunction with a volume) have also been assigned to adata bank 110, however, physical sectors of the disks 252 of the datastores 250 of the data banks may not have yet been assigned to store thedata corresponding to those segments. Thus, at step 360 physicalsegments of disks 252 on the data bank 110 to which a logical segment ofthe volume has been assigned may be determined and assigned to thelogical segments. This segment mapping may be stored in the local tables245 of each data bank 110. This assignment may, as mentioned earlier,take place at some later point, for example, when a command firstattempts to write a logical segment.

When making this assignment, in one embodiment the areas differentperformance characteristics of disks 252 may be accounted for relativeto the accessed logical segment. In other words, disks 252 may havesegments which are more efficiently accessed than other segments of thesame disk. Therefore, in one embodiment it may desirable to assignphysical segments of a disk 252 based upon criteria associated with thelogical segment. The characteristics may include for example, suchthings as a quality of service designation associated with a volumecorresponding to the logical segment, a number of accesses to the volumecomprising the logical segment, etc.

At step 370, then, information corresponding to the volume may bestored, such that the location of segments corresponding to the volume,or segment corresponding to the implementation of RAID in conjunctionwith the volume, may be determined from this stored information. Thisstored information (collectively referred to as mapping data) maytherefore include an identification for the volume, the randompermutation corresponding to the volume (for example, indicating theorder of data banks 110 on which the segments are located) and theparity group size of any RAID implementation (for example, if the volumecorresponds to a 4+1 RAID set, a 7+1 RAID set, if RAID 1 is implemented,etc.). This data may be stored, for example, in global tables 240 suchthat it can be communicated to other distributed RAID applications 210on other data banks 110 to ensure that at least a portion of the set oftables 240 associated with each distributed RAID application 210 remainssubstantially consistent and the location.

The above description may be better understood with reference to FIG. 4which depicts one example of one embodiment of a distributed RAID systemwith five data banks 110. Here, each data store 250 of each data bank110 has been laid out as a set of equally sized segments 402, which forpurposes of this example will be assumed to be 1 MB in size. Suppose,now that a host 102 requests a volume of 8 MB with RAID level 5 fromdistributed RAID application 210 b on data bank 110 b. In this case,distributed RAID application 210 b may determine that eight segments 402are required for the data portion of the volume while two segments 402segments may be required to store the redundancy data for the volume inconjunction with implementation of the desired RAID level for thevolume. Distributed RAID application 210 b may then determine a randompermutation for the volume. For purposes of this example, assume thatthe random permutation is: data bank 110 b, data bank 110 d, data bank110 a, data bank 110 c, and data bank 110 e. Thus, data bank 110 b maybe assigned segment 402 a, the first segment of the requested volume,data bank 110 d may be assigned segment 402 b, the second segment of therequested volume, data bank 110 a may be assigned segment 402 c, thethird segment of the requested volume, data bank 110 c may be assignedsegment 402 d, the fourth segment of the requested volume and data bank110 e may be assigned segment 402 e, the fifth segment of the requestedvolume. The assignment then begins again with the first data bank 110 ofthe random order such that data bank 110 b may be assigned segment 402f, the sixth segment of the requested volume, data bank 110 d may beassigned segment 402 g, the sixth segment of the requested volume anddata bank 110 a may be assigned segment 402 h, the eighth segment of therequested volume.

Once the data segments 402 a-402 h for storing data associated with thevolume have been assigned, distributed RAID application 210 b may assignsegments 402 for any data associated with the implementation of thatRAID level. In this example, as RAID 5 is desired with respect to thevolume, distributed RAID application 210 b may determine that as fivedata banks 110 are being utilized a (4+1) parity set may be desired.Distributed RAID application 210 b may then determine that to store theparity to implement RAID 5 in conjunction with eight segments 402 anadditional two segments 402 may be needed.

Furthermore, it may be desired that the parity created utilizing aparticular set of data segments 402 will not be stored on a data bank110 having any of those set of data segments 402 in its data store.Thus, distributed RAID application 210 b may also determine a locationwhere each of the parity segments will be allocated based on thedetermined RAID parity group size, the location of the first datasegment 402 a, etc. Here, parity segment 402 i which will store theparity data corresponding to the data stored in data segments 402 a, 402lb, 402 c and 402 d will be allocated in data store 250 c of data bank110 c while parity segment 402 j which will store the parity datacorresponding to the data stored in data segments 402 e, 402 f, 402 gand 402 h will be allocated in data store 250 e of data bank 110 e.Notice here that the parity segments 402 i, 402 j which will store theparity information associated with the implementation of RAID inconjunction with the volume comprising data segments 402 a-402 h arelaid out and sized substantially identically to as those segments 402a-402 h which store the data associated with the volume.

Thus, when a host 102 accesses the volume, a request with a logicaladdress corresponding to the first data segment of the volume maycorrespond to data segment 402 a on data bank 110 b, a request with alogical address corresponding to the second data segment of the volumemay correspond to data segment 402 b on data bank 110 d, etc. Noticehere, that the allocated data segments 402 a-402 h may reside ondifferent data banks 110 and that the location of any allocated datasegment 402 a-402 h may be determined using the random permutationassociated with that volume (for example, as stored in global tables 240at data banks 110). As discussed above, however, data stores 250 on databanks 110 have been virtualized, thus the requesting host may not beaware of the location of the data segments 402 in data stores 250, thatmultiple data stores 250 exist, that data stores 250 are spread acrossmultiple data banks 110, etc. Host 102 believes it is addressing asingle contiguous volume.

It will be apparent that the location of the data segments 402 on databanks 110 (and the corresponding random permutation of data banks 110)in this example is for purposes of illustration and that the datasegments 402 of a volume may be located on any of data stores 250 on anyof the data banks 110 according to almost any random, or other,permutation. Furthermore, it will be noted that while each of segments402 is in this example 1 MB, these may be of any size without loss ofgenerality and that a 1 MB size has been chosen solely for ease ofillustration.

As can be seen from the above description then, the location of aparticular data segment 402 or parity segment 402 can be determinedalgorithmically (for example, using the same random permutation used toassign segments for the volume, locate the parity segments for thevolume, etc.) using the random permutation associated with the volumeand the RAID parity group size. Thus, the information may be stored inconjunction with an identification corresponding to the volume, forexample in set of global tables 240. Furthermore, these global tables240 may be communicated between data banks 110, or otherwise updated,such that at least portions of the set of global tables 240 in each ofthe data banks 110 may be kept substantially consistent.

It may be helpful here to briefly delve into more detail regardingglobal tables 240 associated with distributed RAID application 210. Asdiscussed, in one embodiment, global tables 240 may store informationassociated with volumes created by distributed RAID application 210where those tables 240 can be used to determine a data bank 110associated with a data segment within that volume or where a paritysegment associated with a data segment corresponding to that volume islocated. Global tables 240 may therefore comprise a set of tables, eachtable corresponding to a volume implemented with respect to databanks110. In particular, one of these tables 240 may contain data which maybe used to identify a data bank 110 whose data store 250 comprises acertain segment of a volume. Specifically, this table may be used tocorrelate a logical address associated with a volume with the data bank110 where the segment (data, redundancy, etc.) corresponding to thatlogical address is stored.

FIG. 5 depicts a graphical representation of one embodiment of this typeof table, where each volume may have an associated instance of such atable associated. Table 550 includes entries for LV number 504, segmentsize 508, segment count 512, quality of service (QOS) 514, range count518, information for range entries, including in the embodiment depicteda first range 524 a and a second range 524 b and any additional rangeentries 524 n.

LV number 504 is a unique number used to identify a particular volume,segment size 508 corresponds to the size of the segments used toimplement the volume, segment count 512 corresponds to the number ofsegments corresponding to the logical volume (for example, both thenumber of data segments and redundancy segments, just the number of datasegments, etc), QOS 514 indicates the quality of service which it isdesired to implement with respect to the volume (note that this QOSindicator may indicate a priority to be given to that volume relative toother volumes stored on data banks 110) and range count 518 indicates anumber of ranges associated with the volume, while range entries 524each correspond to one of those ranges.

A range may correspond to a particular data bank 110 order and RAIDimplementation. Multiple ranges may be utilized to implement a volumefor a variety of reasons. Specifically, for example, multiple ranges maybe utilized in conjunction with a volume because different data stores250 at different data banks 110 may have different amounts of storage indata store 250 available for use. This may lead to a situation where forexample, for a first range of a volume all data banks 110 may beutilized in conjunction with a first RAID implementation while in asecond range of a volume fewer than all the data banks 110 available maybe utilized in conjunction with a second RAID implementation (where thefirst and second RAID implementations may, in fact, be different levelsthan one another). Each of these ranges may therefore correspond tosegments laid out according to different data bank 110 orders (forexample, random permutations, etc.), having a different number of databanks 110 available for use, a different type of RAID, etc.

To illustrate using a concrete example, brief reference is made back toFIG. 4. Suppose that the volume of 8 MB with RAID level 5 is laid out asshown, where the data segments are laid out according to the order databank 110 b, data bank 110 d, data bank 110 a, data bank 110 c, and databank 110 e and RAID 5 is implemented in conjunction with the volumeutilizing a (4+1) parity set may be desired with the parity segmentsassigned in data store 250 c of data bank 110 c and data store 250 e ofdata bank 110 e.

Now suppose that it is requested to add an additional 3 MB to thisvolume. However, suppose in this instance that data stores 250 of databanks 110 e, 110 c and 110 d have no more room. Thus, in this case theonly solution may be to allocate the additional desired 3 MB betweendata banks 110 a and 110 b which have remaining storage in data stores250. Furthermore, as only two data banks 110 may be available for use itmay only be possible to utilize a RAID level of 1 instead of RAID 5 asutilized with the first 8 MB of the volume. Thus, in this case the first8 MB of the volume may correspond to a first range, and have a firstrange entry in a table corresponding to the volume with a first set ofvalues while the next 3 MB of the volume may correspond to a secondrange, and have a second range entry in a table corresponding to thevolume with a second set of values. As may be apparent after readingthis disclosure, this type of occurrence may occur with some frequency.

Returning to FIG. 5, to deal with these types of situations, amongothers, each range of a volume may have an entry in a table 550 suchthat the location of segments in that particular range may be determinedfrom the range entry corresponding to that range. Entries 524 for eachof the ranges of the volume corresponding to the table 550 areassociated with range count 518. In one embodiment, range count 518 maycorrespond to the number of ranges of a volume such that the number ofrange entries 524 corresponds to the range count 518. While only rangeentries 524 a and 524 b are shown it will be noted that the number ofrange entries 524 in a table will depend on the number of rangescorresponding to the volume to which that table corresponds. Thus, if avolume is divided into three ranges, there will be three range entries524 in table 550 such that there is a range entry 524 comprisinginformation for each range of the volume corresponding to table 550.

Information for a range entry 524 includes type 526, start 530, end 534,network RAID 538, network RAID size 542, disk RAID 546, disk RAID size550, databank count 554, databank order 558 and a disk count 562 anddisk order 566 corresponding to each data bank 110 used to storesegments associated with range 524 (in other words there will be a diskcount 562 and disk order 566 equal to databank count 554 of that rangeentry 524). Type 526 describes the type of the range corresponding toinformation for range entry 524: for example, normal, source (SRC),destination (DST) or other type of range. Start 230 is the first logicalsegment address of the range of the volume corresponding to range entry524. End 234 is the last logical segment address of the rangecorresponding to information for the range of the volume correspondingto range entry 524. Other arrangements are also possible, for example,end 524 may be a count which is the maximum number of segments or blocksin the range, etc.

Databank count 554 may correspond to the number of data banks 110 onwhich the range corresponding to the range entry resides, databank order558 may be the order in which segments in that range were assigned todata banks 110 while network RAID 538, network RAID size 542, disk RAID546 and disk RAID size 552 may correspond to the type of RAIDimplemented in conjunction with the range of the volume corresponding torange entry 524.

Network RAID 538 is the type of RAID being implemented in associationwith the volume corresponding to the table 550, for example, RAID 0,RAID 1 or RAID 5 or other RAID types. Network RAID Size 542 is theparity group size of the RAID type used in the range. The Network RAIDSize 542 may be limited by the number of data banks 110 in the range tobe less than or equal to the number of databanks in the rangecorresponding to information for range 524. Disk RAID 546 is the type ofRAID being implemented across disks in the databanks in the range. DiskRAID size 552 may be the parity group size of the RAID type used acrossthe disks 252 in the data store 250 of each data bank 110 and may belimited to be less than or equal to the number of disks in the databank.In embodiments, RAID across the disks in the databanks 110 in the rangeis optional and may or may not be used. In such embodiments, either DiskRAID 546, Disk RAID Size 552 or both may not be used or may be omitted.

Data bank count 554 is the number of databanks in the range and Databankorder 558 is the order in which RAID is implemented (for example,striped) across the data banks 110 in the range. For example, data banks110 may have data corresponding to the logical addresses of the volumesaved in a certain order and databank order 558 corresponds to thisorder. Disk count 562 is the number of disks within a data bank 110 ofthe range and disk order 566 is the order in which RAID is implementedacross disks of a particular databank 110. For example, disks 252 mayhave segments saved to them in a certain order and disk order 566 is theorder in which segments are stored across disks 252 in a data bank 110.Thus, for each databank 110 used to store segments of the rangeassociated with the range entry 524 there will be a corresponding diskcount 562 and disk order 566 (in other words the number of disk counts562 and disk orders 566 will, in one embodiment, be equal to databankcount 554 of that range entry 524). In embodiments, RAID across disks252 in the data banks 110 is optional and may not be used. It will benoted that while table 550 has been described with specificity, thisdescription is by way of example, not limitation and other forms oftable 550 may be utilized. For example, a virtual table may be usedinstead of table 550 and may explicitly list the segment 402 and databank 110 corresponding to each logical address.

Thus, as discussed earlier, information in table 550 may be used toidentify a data bank 110 comprising a data segment 402 corresponding toa logical address (referenced by a host 102 in a command or in any othercontext). For example, knowing the size of segments 402 and using start530, end 534, the range entry 524 corresponding to the address, etc.,the particular data bank 110 corresponding to a logical address of thevolume can be determined.

While one or more portions of tables 240 may be substantially identicalacross all data banks 110 and may describe one or more logical volumeswhich span one or more data banks 110 as described above, other tables245 on a data bank 110 may be distinct to the data bank 110 to which itcorresponds (for instance, table 245 may be unique to the data bank 110on which the corresponding distributed RAID application 210 isexecuting). This table 245 may comprise data pertaining to each disk 252contained in the data store 250 of the corresponding data bank 110 andmay comprise information on where information is stored on or amongdisks 252 of the data store, for example, the sector of a disk 252 wherea segment 402 assigned to the data bank 110 is located in data store250.

In FIG. 6 a graphical representation of one embodiment of this type oftable is depicted. Table 660 may be stored at a particular data bank 110and comprise multiple disk tables 670, each of the disk tables 670corresponding to a disk 252 of the data store 250 within that data bank110 and listing the location of the segments stored within that disk252. More specifically, in most cases disks 252 are divided intophysical sectors, each physical sector having a corresponding address orrange of addresses.

A disk table 670 may be a mapping table which can be utilized todetermine the location of a sector of a disk 252 of the data bank 110where a segment of a volume is stored. Thus, using a table 670 theaddress of a sector on a disk 252 corresponding to a segment of a volumecan be determined. Furthermore, the table may contain one or more flagsor descriptive bits per entry corresponding to a segment or sector ofthe disk, describing the sector or segment stored at that sector.

Referring now to FIG. 7, a graphical representation of one embodiment ofa disk table 670 is depicted. Disk table 670 has multiple entries, eachentry corresponding to a physical segment of the corresponding disk suchthat the entries of disk table 670 describe the physical segments of thedisk 252. Each entry in disk table 670 may also include one or moreflags or bit fields describing the physical segment or segment of thevolume stored at the corresponding sector. More particularly, as shownin FIG. 7, in one embodiment entries in disk table 670 include fieldsfor a logical volume (LV) number, logical segment number, address spaceand sector state. LV number identifies the logical volume to which datastored at that physical segment corresponds. Logical segment numberidentifies the segment of the logical volume corresponding to that data.Address space identifies the segment stored as ‘data’ or ‘redundancy’. Avalue of ‘data’ may indicates that data is stored at the sectorrepresented by the entry, whereas a value of ‘redundancy’ indicates thatthe information stored at the sector may be used for RAID dataprotection and, depending upon the RAID level, may be redundant data,mirrored data or parity information. Sector state indicates the state ofthe segment as being ‘allocated’, ‘zeroed’ or ‘dirty’. ‘Allocated’indicates the segment has been allocated and may comprise valid data.‘Zeroed’ indicates the segment has been zeroed out by writing zeros tothe segment and ‘dirty’ indicates the segment may comprise garbage areotherwise unusable or undesirable values, for example because thesegment has not been zeroed out or allocated, may be storing random bitsor data. In one embodiment, for example, for a new disk all segments ofthe disk may be marked as dirty in a disk table corresponding to the newor newly added disk.

After reading the above description of the tables it will be apparentthat distributed RAID application 210 may utilize the global tables 240to determine which segment corresponds to a logical address of a volume,on which data bank 110 segments corresponding to a volume (either dataor redundancy segments) are located, which segment of a volumecorresponds to a logical address of a volume, where RAID data (paritydata, mirror data, other types of redundancy data, etc.) associated witha segment of a volume is located, which disk 252 on a particulardatabank 110 comprises a segment or other information regarding volumes,segments, or disks 252 corresponding to that particular data bank 110 orother information regarding volumes, segments 402, data banks 110, RAIDdata, etc.

Similarly, distributed RAID application 210 on each individual data bank110 may use local tables 245 on that data bank 110 to determine where onthat data bank 110 (which sector(s) of disk 252, etc.) a particularsegment is located or other information regarding volumes, segments, ordisks 252 corresponding to that particular data bank 110.

After a review of the above disclosure it will be noted that when a host102 issues a commend (such as a READ or a WRITE) to a data bank 110there is certain probability that the data bank 110 which receives thecommand will not comprise the segment referenced by the command. Thus,in order to execute a command, in many cases the data bank 110 whichinitially received the command will need to forward the command toanother data bank 110 or send or receive other commands from other databanks 110. These communications may consume a certain amount ofbandwidth and processing power which could be otherwise utilized.

To reduce communications between data banks, and thereby improveperformance, it may be desired to direct commands from a host to anappropriate data bank (in other words a data bank which may comprisedata utilized in the execution of the command, either because the databank stores data pertinent to the execution of the command or for someother reason). To that end, certain embodiments of the present inventionmay employ methods and systems which allow a command issued by a host tobe routed to a particular data bank.

Specifically, in one embodiment, a table may be kept at a host, wherethe table may comprise a subset of data similar to global tables storedat data banks. Thus, applications on a host responsible for routingcommands issued by host, or utilized in conjunction with such routingapplications, may employ this table to route a command to an appropriatedata bank. One example of such routing applications are themulti-pathing input/output (MPIO) device drivers which may be employed,for example, by Microsoft Windows® compatible hosts and certainembodiments will thus, be illustrated using MPIO as an example. Itshould be understood, however, that other embodiments may be equallywell utilized with outer device drivers or routing applications on ahost, for example, a device mapper in Linux, etc.

It may be helpful here to refer for a moment back to FIG. 4, where forpurposes of this example it will be assumed that each of the hosts 102are Microsoft Windows® compatible. Hosts 102 may direct commandsaddressed to logical addresses to certain internal device drivers andrely on such device drivers to route commands to the particular databanks 110 which correlate with the logical address associated with thecommand. Microsoft Windows® compatible hosts 102 can use multi-pathinginput/output (MPIO) device drivers 113 to route commands to data banks110. However, in many cases, because of the distributed RAID implementedwith respect to data banks 110, MPIO device drivers 113 may routecommands to an inappropriate data banks 110.

MPIO 113 may, however, have the ability to incorporate or utilize DeviceSpecific Modules (DSM) to integrate MPIO 113 with specific hardware andprovide the ability to implement vendor unique commands. Thus,embodiments of the present invention may employ a DSM 115 residing oneach host 102 in conjunction with MPIO 113 on the host 102 in order todetermine an appropriate data bank 110 for the routing of a command androuting the command to that data bank 110.

The use of such a DSM 115 may be illustrated better with respect to FIG.8 which depicts one embodiment of a method for initializing, updatingand utilizing a DSM at a host 102. At step 810, when a DSM 115 on a host102 is initialized, whether because a host 102 is booted or powered on,the DSM 115 is installed for the first time, etc., the DSM 115 mayregister with the MPIO. As is known in the art, registering with an MIPO113 may entail providing the MPIO with information to identify whichcommands received by the MPIO 113 should be processed in conjunctionwith the DSM 115.

At step 820 the DSM 115 may then receive a command from the MPIO 113based on the data provided to MPIO 113 during registration of the DSM115, such that when the MPIO 113 receives such a command it is passed toDSM 115 and MPIO 113 routes the command, if needed, based upon adetermination made by the DSM 115. In one embodiment, the command maycorrespond to a particular volume which may be associated with the host102. At step 830, then, the DSM 115 may determine if a tablecorresponding to that volume is stored on the host 102 and accessible bythe DSM 115. If not, at step 840 the DSM 115 may obtain a tablecorresponding to that volume from a data bank 110.

In order to obtain this table, the DSM 115 may send a command to a databank 110 to obtain the table corresponding to the volume. This commandmay be a vendor specific SCSI command which requests that a table forthe specific volume be returned to the DSM 115 at the host 102 fromwhich the vendor specific command was issued. The distributed RAIDapplication 210 at the data bank 110 may receive this command from theDSM 115 at the host 102, assemble the desired table for the volumespecified by the received command and send the assembled table back tothe host 102 which issued the command so that this table for that volumemay be stored at the host 102 and accessed by the DSM 115.

Returning to step 830 if the DSM 115 does have a table for the volumecorresponding to the command received by the DSM 115 at step 850 the DSM115 may check to see if a flag corresponding to that volume has beenset. This flag may indicate that the table corresponding to the volumeis out of sync with the data in global table 240 corresponding to thatvolume on data banks 110 and thus the table at the host 102 may need tobe updated. The setting of this flag will be explained in more detaillater.

To continue, if a flag corresponding to the volume has been set at step850 the DSM 115 may, at step 860, update the table corresponding to thatvolume by sending a command to a data bank 110 to obtain the tablecorresponding to the volume in the same manner as that discussed abovewith respect to step 830. If there is no set flag corresponding to anout of sync table for that volume, a data bank 110 where the commandreceived by DSM 115 should be routed may be determined at step 870.

At this point, an overview of a table corresponding to a volume storedat host 102 and accessible by DSM 115 may be useful. Turning briefly toFIG. 9 which depicts a representation of one embodiment of such a table.Table 917 can include selected subsets of information contained in atable 550 corresponding to the same volume. More particularly, table 917can include map version 903, LV number 904, segment size 908, segmentcount 912, range count 918, etc. and range entries 924 similar to thosediscussed above with respect to table 550. Each range entry 924 of table917 may therefore include start 930, end 934, data bank count 954, databank order 958, etc.

Referring back now to step 70 of FIG. 8, DSM 115 can utilize a logicaladdress of a volume reference by a command and the table correspondingto the volume to mathematically determine a data bank 110 where thecommand should be routed. More specifically, the logical address may beused in conjunction with starts 930 and ends 934 and service suchcommands as described herein. To service, for example, WRITE and READcommands from hosts 102, DSM 115 can map logical addresses to data banks110 according to table 917. DSM 115 can examine starts 930 and ends 934in table 917 to determine within which range 924 particular logicaladdresses lie. More particularly, DSM 115 can determine

a modulo b=c   Eq. 1

Where a is a particular logical address, b is data bank count 954, and cis data bank order 958. Using the results of Eq. 1, DSM 115 can look uptargeted data bank 110 at the position in data bank order 958 returnedby Equation 1.

To illustrate using a particular example, assume a WRITE commandaddresses a particular logical address 3503 and table 917 contains thefollowing entries:

Field Value Map version 1 LV number 1 Segment size 512 Segment Count2048 Range count 4 Range 924a Start 0 End 999 Data bank count 5 Databank order 1, 3, 2, 4, 5 Range 924b Start 2000 End 2999 Data bank count5 Data bank order 1, 3, 2, 4, 5 Range 924c Start 3000 End 3999 Data bankcount 5 Data bank order 1, 3, 2, 4, 5 Range 924d Start 4000 End 5000Data bank count 5 Data bank order 1, 3, 2, 4, 5

In this example, DSM 115 can examine starts 930 and ends 934 until itlocates start 930 and end 934 for range 924 c between which logicaladdress 3503 lies. Thus, DSM 115 uses range 924C elated data bank count954 equal to 5 to determine:

3503 modulo 5=3   Eq. 2

From Eq. 2, DSM 115 determines that the particular WRITE commandaddressed to logical address 3503 should be directed to third data bank110 indicated by data bank order 958 equal to the order list 1, 3, 2, 4,5. Because data bank 2 appears third in data bank order 958 (note thatdata bank 2 and 3 are in reverse numeric order), DSM 115 determines thatthe particular WRITE command to logical address 3503 should be mapped todata bank 110 b (the second data bank). DSM 115 can map READ or othercommands in a similar manner.

After determining a proper data bank 110 where the command should berouted at step 870 the DSM 115 may return an identifier for that databank 110 (for example, a path or other identifier) to MPIO 113 such thatthe command may be routed to the determined data bank 110 at step 880.

A response to the command from the data bank 110 may then be received atstep 882. It can then be determined if that response is a SCSI “unitattention” at step 890. In one embodiment, in response to a change in avolume (for example, portions of a volume have been moved, the RAIDlevel implemented with respect to the volume changed, etc.) thedistributed RAID application 210 may set a flag corresponding to thevolume indicating that the global table 240 corresponding to the volumehas been changed. Thus, if a command for that volume is received from ahost 102 at a data bank 110 before responding to that commanddistributed RAID application 210 may determine if a flag indicating thatthe global table 240 corresponding to that volume has been set. If not,distributed RAID application 210 may respond to the received command. Ifthe flag is set, however, distributed RAID application may respond tothe received command with a “unit attention” indicating that the globaltable 240 corresponding to that volume has changed.

Thus, at step 890 if the received response is a “unit attention” the DSM115 may set a flag at step 894 indicating that the table correspondingto the volume is out of sync with the data in the corresponding globaltable 240 on data banks 110 and thus may need to be updated. Otherwise,at step 892 the response may be passed to MPIO 113 for returning to thehost 102.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofinvention.

For example, other embodiments of systems and methods utilized to routecommands may be implemented in conjunction with the Linux operatingsystem utilizing a device mapper. This device mapper may be a processthat wakes up periodically to determine if tables at data bank 110corresponding to a volume associated with the host 102 are needed orhave been changed, acquire these tables and reconfigure device tables inthe operating system kernel according to any of these obtained tables.

Another example pertains to the synchronization of information in tablesat the host and those at data banks. In some embodiments, DSM can verifythat a table is current by issuing a SCSI inquiry command to any databank. Data banks can be configured to respond to a SCSI inquiry commandby returning the table corresponding to the volume currently in use bydata bank to host. Both tables at host and data bank can contain a mapversion. Distributed RAID application can increment the map version inthe table at the data bank corresponding to the volume when values inthe table change. When DSM receives the table from data bank, DSM cancompare map versions from the table received and the table stored at thehost. If the map versions differ, the DSM can update the table for usein routing commands from host.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any component(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or component of any or all the claims.

1. (canceled)
 2. A system for implementing distributed RAID, comprising:a set of hosts, each host comprising: a processor; a first computerreadable medium, comprising instructions executable for: obtaining afirst command corresponding to a first segment of the first volume;determining a first storage appliance of a plurality of storageappliances, wherein a first storage appliance corresponds to a firstsegment of a first volume; and routing the command to the first storageappliance, wherein each of the set of hosts is coupled to a distributedRAID system comprising the plurality of storage appliances, each storageappliance coupled to each of the other plurality of storage appliances,each storage appliance including: a processor; and a data store; asecond computer readable medium comprising instructions executable for:implementing commands in conjunction with the first volume, wherein thefirst volume comprises a set of segments including the first segment andthe set of segments of the first volume are stored on the plurality ofstorage appliances according to a first storage appliance order wherethe first storage appliance order is a first ordering of the pluralityof storage appliances including each of the plurality of storageappliances exactly once, and wherein redundancy data corresponding to afirst RAID level implemented in conjunction with the first volume isstored on each of the plurality of storage appliances, such that foreach segment of the volume a corresponding redundancy segment comprisingthe redundancy data associated with that segment of the volume does notreside on the same storage appliance as the corresponding segment of thevolume.
 3. The system of claim 2, wherein determining the first storageappliance is done by a device specific module (DSM).
 4. The system ofclaim 3, wherein the DSM comprises a first table associated with thefirst volume comprising the first storage appliance order.
 5. The systemof claim 4, wherein a second volume is stored on the plurality ofstorage appliances according to a second storage appliance order, wherethe second storage appliance order is a second ordering of the pluralityof storage appliances including each of the plurality of storageappliances exactly once.
 6. The system of claim 5, wherein the DSMcomprises a second table associated with the second volume comprisingthe second storage appliance order.
 7. The system of claim 2, whereinthe first table is updated based on a notification received from one ofthe plurality of storage appliances that the table has changed.
 8. Thesystem of claim 7, wherein the notification is sent from the one of theplurality of storage appliances when a threshold amount of dataassociated with the first volume has been moved.
 9. The system of claim7, wherein the first computer readable medium further comprisesinstructions for updating the table by obtaining the table from one ofthe plurality of storage appliances.
 10. A method for routing commandsfrom a host computer to a distributed RAID system, comprising: obtaininga first command corresponding to a first segment of the first volume;determining a first storage appliance of a plurality of storageappliances, wherein a first storage appliance corresponds to a firstsegment of a first volume; and routing the command to the first storageappliance in a distributed RAID system comprising the plurality ofstorage appliances, each storage appliance coupled to each of the otherplurality of storage appliances, and each storage appliance configuredfor: implementing commands in conjunction with the first volume, whereinthe first volume comprises a set of segments including the first segmentand the set of segments of the first volume are stored on the pluralityof storage appliances according to a first storage appliance order wherethe first storage appliance order is a first ordering of the pluralityof storage appliances including each of the plurality of storageappliances exactly once, and wherein redundancy data corresponding to afirst RAID level implemented in conjunction with the first volume isstored on each of the plurality of storage appliances, such that foreach segment of the volume a corresponding redundancy segment comprisingthe redundancy data associated with that segment of the volume does notreside on the same storage appliance as the corresponding segment of thevolume.
 11. The method of claim 10, wherein determining the firststorage appliance is done by a device specific module (DSM).
 12. Themethod of claim 11, wherein the DSM comprises a first table associatedwith the first volume comprising the first storage appliance order. 13.The method of claim 12, wherein a second volume is stored on theplurality of storage appliances according to a second storage applianceorder, where the second storage appliance order is a second ordering ofthe plurality of storage appliances including each of the plurality ofstorage appliances exactly once.
 14. The method of claim 13, wherein theDSM comprises a second table associated with the second volumecomprising the second storage appliance order.
 15. The method of claim10, wherein the first table is updated based on a notification receivedfrom one of the plurality of storage appliances that the table haschanged.
 16. The method of claim 15, wherein the notification is sentfrom the one of the plurality of storage appliances when a thresholdamount of data associated with the first volume has been moved.
 17. Themethod of claim 16, wherein the first computer readable medium furthercomprises instructions for updating the table by obtaining the tablefrom one of the plurality of storage appliances.
 18. A non-transitorycomputer readable medium, comprising instructions executable for:obtaining a first command corresponding to a first segment of the firstvolume; determining a first storage appliance of a plurality of storageappliances, wherein a first storage appliance corresponds to a firstsegment of a first volume; and routing the command to the first storageappliance in a distributed RAID system comprising the plurality ofstorage appliances, each storage appliance coupled to each of the otherplurality of storage appliances, and each storage appliance configuredfor: implementing commands in conjunction with the first volume, whereinthe first volume comprises a set of segments including the first segmentand the set of segments of the first volume are stored on the pluralityof storage appliances according to a first storage appliance order wherethe first storage appliance order is a first ordering of the pluralityof storage appliances including each of the plurality of storageappliances exactly once, and wherein redundancy data corresponding to afirst RAID level implemented in conjunction with the first volume isstored on each of the plurality of storage appliances, such that foreach segment of the volume a corresponding redundancy segment comprisingthe redundancy data associated with that segment of the volume does notreside on the same storage appliance as the corresponding segment of thevolume.
 19. The computer readable medium of claim 18, whereindetermining the first storage appliance is done by a device specificmodule (DSM).
 20. The computer readable medium of claim 19, wherein theDSM comprises a first table associated with the first volume comprisingthe first storage appliance order.
 21. The computer readable medium ofclaim 20, wherein a second volume is stored on the plurality of storageappliances according to a second storage appliance order, where thesecond storage appliance order is a second ordering of the plurality ofstorage appliances including each of the plurality of storage appliancesexactly once.
 22. The computer readable medium of claim 21, wherein theDSM comprises a second table associated with the second volumecomprising the second storage appliance order.
 23. The computer readablemedium of claim 18, wherein the first table is updated based on anotification received from one of the plurality of storage appliancesthat the table has changed.
 24. The computer readable medium of claim23, wherein the notification is sent from the one of the plurality ofstorage appliances when a threshold amount of data associated with thefirst volume has been moved.
 25. The computer readable medium of claim24, wherein the first computer readable medium further comprisesinstructions for updating the table by obtaining the table from one ofthe plurality of storage appliances.